Alloyed materials are a long-studied class of metallic solids with a variety of applications. In the bulk, alloys are described as solid solutions of metals. The “solvent” or “host” is the element present in the greatest amount, and the “solute” is the minor component. At the bulk scale, the formation of the alloy may be understood by considering the overall free energy of metal mixing. Mixing of two metals creates a favorable entropic term. However, the enthalpy of formation may or may not be negative depending on the metals and will often dominate the free energy of formation at low temperatures. Therefore, the majority of bulk alloys are formed by a diffusion-quench process where two metals are mixed at high temperature and then cooled rapidly to “freeze” the combined state. The resulting material compositions are generally described by experimentally-derived phase diagrams and a conceptual framework defined by Hume-Rothery that incorporates impacts of metal atom valency, size, and electronegativity.
It has been known for centuries that alloyed materials can dramatically enhance the properties of their constituent metals, and like their monometallic counterparts, may also exhibit significant changes in their physical properties at the nanometer length scale. From bronze to steel, alloyed materials have defined the technological capabilities of their times, and like their monometallic counterparts, can experience dramatic changes in their physical properties at the nanoscale. Indeed, multimetallic nanoparticles promise to provide improved catalysts for efficient use of fossil fuel resources as well as multifunctional tools in biomedical applications. However, current methods to prepare these particles are typically energy intensive and afford limited tunability of particle composition, especially with respect to particle surface structure. Yet, without well-defined architectures, the promise of this material wellspring cannot be achieved.
One particularly interesting class of these materials is small metal nanoparticles (diameter (d)≅2-5 nm) which exhibit properties different from both metal clusters (<200 atoms) and larger metal nanomaterials (d>5 nm). These “few nm” particles display both unique catalytic and optoelectronic behaviors. For example, gold, silver, and copper nanoparticles all exhibit photoluminescence (PL). Gold nanoparticles exhibit PL emission throughout the visible and near-infrared (NIR) depending upon their size, shape, and surface chemistry. Yet, aspects of the PL from metallic nanostructures are not well understood and proposed mechanisms differ for gold nanoparticles (AuNPs) that exhibit localized surface plasmon resonances (LSPRs) and small gold nanoparticles, which do not.
In anisotropic nanoparticles that exhibit an LSPR, PL is observed between ˜630-750 nm, and is attributed to emission from the plasmon band. As the diameter of the nanoparticle decreases below 3 nm, the LSPR of the AuNPs no longer manifests, and instead these particles exhibit PL in the NIR region with a large shift (˜600 nm) between their absorption and emission wavelengths. Further decreases in particle size result in a hypsochromic shift of the NIR emission and an increase in quantum yield (Φ). Photoluminescence from these small AuNPs is attributed to emission from surface states, since experiments have shown that the number of gold(I)-thiolate bonds on the particle surface is well-correlated with observed PL intensity. Because the NIR PL of AuNPs occurs only at small diameters, the ability to tune this optoelectronic behavior has been limited. However, tuning of the particle composition within the same size range could allow for increased versatility in this class of nanomaterials, and present a new perspective on the underlying PL phenomena of small metal nanoparticles.
For high surface area alloys, and in particular nanoparticles, surface composition is a critical parameter in the function of the material. The significance of both the formation and architecture of the nanoparticle surface derives from a canonical difference between bulk and nano scale materials: the surface to volume atom ratio. In applications such as catalysis, the large number of surface atoms per unit mass is preferred because it improves the economy of the catalytic cycle (i. e. there are more active catalyst sites per unit mass). However, large surface areas pose a challenge for the synthesis, stabilization, and rational design of nanoparticle alloys. For small alloys (d=1-5 nm), the mixing behavior of two metals is dominated by a parameter termed “surface segregation energy” (SSE) and is reported as a function of metals mixed and crystal facet exposed. SSE defines the change in energy to exchange a solute atom in the bulk host with a surface atom of the host crystal.
According to the present disclosure, preferred methods are presented for the synthesis of small, discrete gold-copper nanoparticle alloys with tunable compositions from 0-100% molar ratio Cu (d≅2 to 3 nm). The resulting materials display some of the first observations of composition-driven, tunable PL in the NIR region. To synthesize these nanoalloys according to preferred aspects of the present disclosure, the molar ratio of metal precursors may be adjusted in order to mediate final metal molar ratio in the resulting nanoparticle.
In accordance with the present disclosure, the SSE surface energy of the nanoparticles may be modulated in order to tune separation behavior in colloidal nanoparticle alloys. This preferred approach modulates the surface segregation energies of two metals in solid solution by using organic ligands to modify the surface energy of the constituent metals via coordination. These new methods according to preferred aspects of the present disclosure, as well as the resulting material library, should greatly expand the utility of small nanoparticles and provide the clarity necessary to implement these multifunctional platforms. In accordance with the present disclosure, a series of Au—Co, Au—Ni, Au—Cu, Au—Fe, Au—Ag, and Au—Zn nanoparticles have been synthesized, each with controlled molar ratios of the solute metal. This composition control has led to the discovery of a new class of NIR emitters. These results have impact for the catalysis, bioimaging, and nanomedicine communities.